1. Field of Invention
The present invention relates to improved ultra-wideband synthetic aperture radar and aircraft antennas therefore. The exploitation of the radio frequency spectral content including, for example, VHF/UHF bandwidths of a decade or more, of synthetic aperture generated images using airborne platforms of limited size, is disclosed. Antenna polarization agility may be provided by a log periodic tripole array that provides low aerodynamic drag, robust aerodynamic stability, and uniform gain over its bandwidth without the need for a radome.
2. Description of Related Art
Airborne synthetic aperture radar has historically been implemented over frequencies ranging from perhaps 10 MHz to around 100 GHz with bandwidths from a few MHz to several hundred MHz. Ultra-wideband synthetic aperture radar has generally been defined as radar with a bandwidth of at least +−25% of center frequency. An example would be a system operating from 200 to 400 MHz. Ground penetrating and foliage penetrating SAR generally performs best at the lowest frequency that will provide adequate spatial resolution, resulting in systems generally falling in the VHF and UHF bands. For detection of small shallow buried mines, somewhat higher frequencies of up to several GHz (corresponding to the resonant scattering of the emplaced mines) have been successfully used.
Foliage penetration synthetic aperture radar (FOPEN radar) has been implemented in the form of the CARABAS 1 system using a pair of horizontal inflatable antenna elements extending to the rear of the aircraft, and as CARABAS II using forward extending horizontal elements. The CARABAS III system is reported by Jane's to operate in the 20 MHz to 90 MHz band. Alternatively, log periodic monopole arrays extending horizontally aft of the wing trailing edge may be used, however, such arrangements may require elaborate mounting systems in order to accommodate flap and aileron articulation and are restricted to horizontal polarization. Under-wing log periodic dipole arrays have been used, utilizing the bottom of the wing as a reflective ground plane. This results in dipole length being limited by wing chord dimensions and is also limited to horizontal polarization. The exclusive use of horizontal polarization may result in non-detection of generally vertically oriented targets, such as standing or walking people. Ground penetration is also severely limited when using exclusively horizontal polarization.
U.S. Pat. No. 4,866,446 and U.S. Pat. No. 4,965,582, both to Hellsten and both describing several variations of the aforementioned CARABAS systems, disclose synthetic aperture radar systems that only utilize horizontally polarized rf signals. Vertically oriented targets may not be detected due to very limited response to horizontally polarized rf signals. Rf ground penetration needed to detect subsurface targets is inferior in the case of horizontal polarization, a phenomenon familiar to anyone who has used polarized sunglasses to see into a pool of water by suppressing the horizontally polarized light reflected from the surface of the water. A further disadvantage of using only horizontal polarization is that automatic target detection (ATD) is not nearly as effective with single polarization as it is with the three polarization channels (H-H, V-V, V-H). The two additional image layers provided by a fully polarimetric SAR and missing from a single polarization SAR importantly help to discriminate targets from clutter and thereby provide a lower false alarm rate and a lower target detection threshold. The functionality of the prior art systems described in U.S. Pat. No. 4,866,446 and U.S. Pat. No. 4,965,582 is further hindered by the use of a common resolution cell size (in azimuth and in range) over the entire system bandwidth, which limits the resolution needed for detection of small targets which respond to higher frequencies but have a smaller radar cross section than similar but larger targets. Additionally, range resolution is generally proportional to bandwidth and, although Hellsten discloses systems utilizing widely separated frequency bands, the bandwidth of the individual frequency bands is limited and therefore is also the range resolution of targets responding to those frequencies.
Vertical log periodic monopole arrays have been implemented using tensioned cable elements suspended between the vertical and horizontal stabilizers of an aircraft however; the suspended cable antenna elements are subject to uncontrolled deflections and resulting phase errors, while overall antenna dimensions and orientation is restricted by the aircraft configuration.
Conventional SAR systems generally utilize range compression of the scattered and received return from each chirp (or equivalent) to obtain a temporal (fast time) sequence of the collective contributions of the impulse responses of the various scatterers in the radar scene. With reference to the phase position (in time and space) of each transmitted chirp (or equivalent), the magnitude and phase history of the received and range compressed signal is used to generate a single image for each polarization such as H-H, V-V, and V-H. Any information regarding spectral content contributing to each resolution cell is lost in this process with the exception of delayed returns from resonant targets which may in some cases reveal spectral content in the form of periodic range spreading of the images of resonant targets.
Radar transmitters with instantaneous bandwidths of perhaps 20 MHz to 1000 GHZ may presently be configured from commercial off-the-shelf (COTS) digitally controlled arbitrary waveform generators in conjunction with ultra-wideband power amplifiers. Alternatively, COTS ultra wideband pulse generators may be used (with or without amplifiers) as transmitters. Ultra-wideband radar receivers may similarly be configured using COTS low noise amplifiers in conjunction with COTS high speed analog-to-digital conversion devices and high speed digital processors. Furthermore, the small size, low weight, and low power consumption of currently available high speed processors would make it possible, with sufficient system bandwidth, to simultaneously form images from multiple spectral and polarimetric channels. The result of these developments is that available ultra-wideband synthetic aperture radar performance is becoming limited by available antennas. Furthermore, the images that would result from coherently combining the contributions from signals at extremely diverse wavelengths cannot preserve variation in target phase center as, a function of frequency, cannot take advantage of the superior azimuth resolution otherwise available from the small antenna aperture that should be used for the shorter wavelengths and, except for the forementioned spatially filterable resonance artifacts, neither preserves nor reveals any of the spectral content of the scene.
This is especially true when the antenna selection is narrowed down to those that may be fitted to unmanned Aerial vehicles (UAVs), which are generally smaller than conventional manned aircraft. Conventional dual polarized antennas with acceptable gain at VHF frequencies are undesirably large and may not be able to be carried on small unmanned aerial vehicles (UAVs). Great progress has been made in the field of phased array antennas capable of electronic beam steering. Progress has likewise been made in the filed of conformal (conforming to the aircraft surface) antennas. These developments are well suited for covering one to perhaps a few relatively narrow frequency bands from any single antenna. These recent developments do not readily and economically provide frequency coverage, for example, from 50 MHz to 1000 MHz, with uniform and sufficient gain for synthetic aperture radar use over this entire bandwidth with an acceptable VSWR on a small aircraft. The use of large arrays of active antenna elements may result in an expensive system with high electrical power consumption.
Conventional log periodic dipole arrays (LPDAs) are effective over bandwidths of up to several decades and are commonly configured for dual (horizontal and vertical) polarization. Conventional dual polarized LPDAs utilize vertical elements that tend to be aerodynamically unstable and exhibit high aerodynamic drag. The forward facing elements would also be subject to aerodynamic instability. A radome may be used to insure aerodynamic stability; however this may only exacerbate the drag problem, especially in the VHF band. Conventional aircraft antennas are commonly configured as a swept blade element in order to achieve aerodynamic stability in conjunction with low drag. Conventional LPDAs cannot be built up of such swept blade elements because half of the elements would be unstable because they would point in the wrong direction (into the wind).
Spectrometric data has been derived from systems using real aperture antennas, but without the much higher spatial resolution provided in accordance with this invention. Inverse SAR (ISAR) has been used to image rotating targets without relying on radar platform movement to generate an aperture. It appears that, as in the case of SAR systems, no ISAR systems have been developed with spectral discrimination capability. Such capability could be extremely valuable for aircraft identification. Infamously, civilian airliners with hundreds of passengers have been shot down for failure to be identified as civilian airliners. Spectrally discriminate ISAR might readily pick up on characteristic resonant features such as fuselage window openings and aid in aircraft identification.